Extensions and Exceptions to Mendels Laws Gene expression appears to alter Mendelian ratios Lethal Genotype: causes death before the individual can reproduce; prevent transfer of his/her genes to the next generation; presence of just 1 copy of the allele results in the death of an individual Humans: miscarriages (spontaneous abortions)/Huntington Disease Incomplete Dominance: Heterozygous phenotype is intermediate between that of either homozygote Snapdragons: Red (RR) Pink (Rr) White (rr) Andalusian Fowl: Black feathers (RR) Blue (Rr) White (rr) Roan Cattle: Brown (RR) Roan (Rr) White (rr) Codominance: when different alleles are both expressed in a heterozygote; ABO blood group (determined by presence of antigens) on RBC (on chromosome 9) 1900s Karl Landsteiner Blood TypeAntigensGenotypes AAIAIAIAIA AAIAiIAi BBIBIBIBIB BBIBiIBi ABA and BIAIBIAIB ONoneii Epistasis: 1 gene masks or affects the expression of a different gene; % of individuals show some degree of expression of the mutant Bombay Phenotype: the presence of a mutant form masks the expression of antigens A and B A and B antigens = carbohydrates bound to lipid protruding from RBC membrane; A and B antigens derived from H substance (2 sugars added); Bombay Phenotype: H substance incompletely formed and not a good substrate for the enzyme that adds a sugar (Type O blood) Penetrance: all or none expression of a genotype Huntington Disease (completely penetrant) Expressivity: refers to severity or extent Pleiotropy: gene controls several functions or has more than 1 effect Porphyria: King George III with an episode relapse 13 years later relapse 3 years later (a single protein affects different parts of the body or participates in more than 1 type of biochemical reaction); problems with the breakdown of hemoglobin; deep red urine, abdominal pain, headaches, vision problems, delirium, muscular weakness, convulsions Marfan Syndrome: defect in elastic connective tissue lens, aorta, bones of limbs and ribs; mutation encoding fibrillin Alkaptonuria: 1 metabolic abnormality blackened urine, problems with muscles and joints, kidney stones, hearing loss Phenocopy: a characteristic that appears to be inherited but is environmentally caused Limb loss from thalidomide; infections Genetic Heterogeneity : when different genes produce the same genotype Hearing loss; clotting disorders Maternal Inheritance and Mitochondrial Genes: Mitochondria with mini chromosome of 37 genes Maternally inherited No crossing over; mutates faster (lacks DNA repair enzymes); high number of free radicals in a confined space Encode proteins used in protein synthesis and energy production Mutations cause great fatigue Myoclonal Epilepsy and Ragged Red Fiber Disease: only affects child of affected mother, not affected father; blotchy red patches, deafness, seizures, brain function problems Lebers Hereditary Optic Neuropathy: sudden bilateral blindness; vision loss at 27 years of age (average) Heteroplasmy: rare condition in which a particular mutation may be present in some mitochondrial chromosomes but not others Expressivity varies among siblings; very severe Linkage: On the same chromosome; No recombination of genes Drosophila melanogastor (Thomas Hunt Morgan): the farther apart 2 genes are on a chromosome, the more likely they are to cross over Led to construction of linkage maps: show order of genes on chromosomes and relative distances between them